This invention relates to salts of organic phosphates useful as transesterification catalysts in melt polymerization reactions of dihydroxy aromatic compounds with diaryl carbonates. The invention further relates to a method for the preparation of polycarbonates using alkali and alkaline earth metal salts of organic phosphates. The method may be used to provide a product polycarbonate comprising a lower level of Fries product than is provided by other known methods employing conventional melt transesterification catalysts.
Increasingly, polycarbonate is being prepared by the melt reaction of a diaryl carbonate with a dihydroxy aromatic compound in the presence of a transesterification catalyst, such as sodium hydroxide. In this xe2x80x9cmeltxe2x80x9d process, reactants are introduced into a reactor capable of stirring a viscous polycarbonate melt at temperatures in excess of 300xc2x0 C. Typically, the reaction is run at reduced pressure to facilitate the removal of by-product hydroxy aromatic compound formed as the diaryl carbonate reacts with the dihydroxy aromatic compound and growing polymer chains.
The Fries rearrangement is a ubiquitous side reaction taking place during the preparation of polycarbonate using the melt process. The resultant xe2x80x9cFries productxe2x80x9d serves as a site for branching of the polycarbonate chains thereby affecting flow and other properties of the polycarbonate. Although, a low level of Fries product may be tolerated in the product polycarbonate produced by the melt process, the presence of higher levels of Fries product may negatively impact performance characteristics of the polycarbonate, such as moldability and toughness. Currently, alkali metal hydroxides, such as sodium hydroxide, are employed as catalysts in the preparation of polycarbonate using the melt process. Alkali metal hydroxides, although effective catalysts in terms of rates of conversion of starting materials to product polycarbonate, tend to produce relatively high levels of Fries rearrangement product. Thus, melt polymerization methodology useful for the preparation of polycarbonate in which the formation of Fries product has been minimized represents a long sought goal among those wishing to practice such methodology.
It would be a significant advantage to prepare polycarbonate by a melt polymerization method which provides high rates of polymerization while minimizing the amount of Fries product formation.
The present invention provides a method for the preparation of polycarbonate, said method comprising contacting at least one diaryl carbonate with at least one dihydroxy aromatic compound in the presence of a transesterification catalyst and optionally a co-catalyst, under melt polymerization conditions to afford a polycarbonate, said transesterification catalyst comprising at least one alkali metal or alkaline earth metal salt of an organic phosphate.
In one aspect the method of the present invention affords a product polycarbonate having a lower level of Fries rearrangement product than polycarbonate of similar molecular weight prepared using a conventional melt transesterification catalyst.
The present invention further relates to a method of preparing polycarbonate under melt polymerization conditions by reacting at least one dihydroxy aromatic compound with at least one diaryl carbonate in the presence of at least one salt of an organic phosphate, said salt having structure I 
wherein A is independently at each occurrence an alkali metal ion or an alkaline earth metal ion; p is an integer having a value of 1 or 2; q is an integer having a value in a range from 1 to about 10, and R1 is an C1-C60 aliphatic radical, a C3-C60 cycloaliphatic radical or a C4-C60 aromatic radical.
The present invention may be understood more readily by reference to the following detailed description of preferred embodiments of the invention and the examples included therein. In the following specification and the claims which follow, reference will be made to a number of terms which shall be defined to have the following meanings:
The singular forms xe2x80x9caxe2x80x9d, xe2x80x9canxe2x80x9d and xe2x80x9cthexe2x80x9d include plural referents unless the context clearly dictates otherwise.
xe2x80x9cOptionalxe2x80x9d or xe2x80x9coptionallyxe2x80x9d means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
As used herein the term xe2x80x9cpolycarbonatexe2x80x9d refers to polycarbonates incorporating structural units derived from at least one dihydroxy aromatic compounds and includes copolycarbonates and polyester carbonates.
As used herein, the term xe2x80x9cmelt polycarbonatexe2x80x9d refers to a polycarbonate made by a process comprising the transesterification of a diaryl carbonate with a dihydroxy aromatic compound in the presence of a transesterification catalyst.
xe2x80x9cBPAxe2x80x9d is herein defined as bisphenol A or 2,2-bis(4-hydroxyphenyl)propane.
xe2x80x9cCatalyst systemxe2x80x9d as used herein refers to the catalyst or catalysts that catalyze the transesterification of the dihydroxy aromatic compound with the diaryl carbonate in the preparation of melt polycarbonate.
xe2x80x9cCatalytically effective amountxe2x80x9d refers to the amount of the catalyst at which catalytic performance is exhibited.
As used herein the term xe2x80x9cFries productxe2x80x9d is defined as a structural unit of the product polycarbonate which upon hydrolysis of the product polycarbonate affords a carboxy-substituted dihydroxy aromatic compound bearing a carboxy group adjacent to one or both of the hydroxy groups of said carboxy-substituted dihydroxy aromatic compound. For example, in bisphenol A polycarbonate prepared by a melt reaction method in which Fries reaction occurs, the Fries product affords carboxy bisphenol A, II, upon complete hydrolysis of the product polycarbonate. 
The terms xe2x80x9cFries productxe2x80x9d and xe2x80x9cFries groupxe2x80x9d are used interchangeably herein.
The terms xe2x80x9cFries reactionxe2x80x9d and xe2x80x9cFries rearrangementxe2x80x9d are used interchangeably herein.
As used herein the term xe2x80x9chydroxy aromatic compoundxe2x80x9d means a phenol, such as phenol, p-cresol or methyl salicylate, comprising a single reactive hydroxy group and is used interchangeably with the term xe2x80x9cphenolic by-productxe2x80x9d.
As used herein the term xe2x80x9caliphatic radicalxe2x80x9d refers to a radical having a valence of at least one comprising a linear or branched array of carbon atoms which is not cyclic. Examples of aliphatic radicals include methylene; ethylene; 1,2-dimethylethylene; hexamethylene; and the like.
As used herein the term xe2x80x9caromatic radicalxe2x80x9d refers to a radical having a valence of at least one, said radical comprising at least one aromatic group. The aromatic radical may be composed entirely of carbon and hydrogen atoms, or may comprise heteroatoms such as nitrogen, oxygen and sulfur.
As used herein the term xe2x80x9ccycloaliphatic radicalxe2x80x9d refers to a radical having a valance of at least one comprising an array of atoms which is cyclic but which is not aromatic. The array may include heteroatoms such as nitrogen, sulfur and oxygen or may be composed exclusively of carbon and hydrogen.
It should be understood that as used herein, the terms xe2x80x9caliphatic radicalxe2x80x9d, xe2x80x9caromatic radicalxe2x80x9d and xe2x80x9ccycloaliphatic radicalxe2x80x9d include both substituted and unsubstituted embodiments of said radicals. Typical substituents according to the present invention for the substituted forms of aliphatic, aromatic and cycloaliphatic radicals include C1-C20 alkyl, C3-C20 cycloalkyl, C4-C20 aryl, halogen, hydroxy, carbonyl, nitro, cyano, C1-C20 alkoxy, C1-C20 alkoxycarbonyl, and the like.
It should be understood that the terms xe2x80x9cmmHgxe2x80x9d and xe2x80x9ctorrxe2x80x9d are used interchangeably herein as units of pressure.
It has been discovered that the use of salts of organic phosphates as polymerization catalysts in the melt transesterification reaction of a dihydroxy aromatic compound such as bisphenol A with a diaryl carbonate such as diphenyl carbonate, provides a product polycarbonate comprising a reduced level of Fries rearrangement product. This reduction in the amount of Fries rearrangement is highly desirable in that it results in increased ductility of the product polycarbonate and avoids uncontrolled branching which may occur at sites of Fries rearrangement. Uncontrolled branching may limit the utility of the product polycarbonate by reducing the ductility of the product polycarbonate. Salts of organic phosphates employed as transesterification catalysts according to the method of the present invention produced less Fries rearrangement product than did alkali metal hydroxide transesterification catalysts, such as sodium hydroxide.
In one embodiment, the present invention provides a catalyst system for the production of polycarbonate under melt polymerization conditions, wherein the polycarbonate has a number average molecular weight, Mn of at least about 8000 daltons and a reduced content of Fries products relative to a polycarbonate of comparable molecular weight prepared using an alkali metal hydroxide transesterification catalyst. In particular, it is desirable to have Fries product of less than 3000 ppm, preferably less than 2000 ppm, more preferably less than 1000 ppm, even more preferably less than 500 ppm.
The transesterification catalyst according to the present invention comprises at least one alkali metal or alkaline earth salt of an organic phosphate, having structure I.
Salts of organic phosphates represented by structure I are exemplified by alkali metal and alkaline earth metal salts of phosphate derivatives of aliphatic alcohols, cycloaliphatic alcohols, and aromatic alcohols. Aliphatic alcohols are illustrated by methanol, glycerol, butanol and octanol. Cycloaliphatic alcohols are illustrated by cyclohexanol, and cyclohexanedimethanol. Aromatic alcohols are illustrated by phenol, bisphenol A, and phenethyl alcohol. Alkali metal and alkaline earth metal salts of phosphate derivatives of aliphatic alcohols, cycloaliphatic alcohols, and aromatic alcohols are illustrated by butanol monophosphate disodium salt, cyclohexhanedimethanol diphosphate tetrapotassium salt, and phenol monophosphate disodium salt.
In one aspect of the present invention structure I represents an alkali metal or alkaline earth metal salt of a naturally occurring alcohol monophosphate derivative. Thus, in one embodiment of the present invention the catalyst is an alkali metal or an alkaline earth metal salt of adenosine 5xe2x80x2-monophosphate, said salt having structure III 
wherein A and p are defined as in structure I. Salts of adenosine monophosphate III are exemplified by adenosine 5xe2x80x2-monophosphate dilithium salt, adenosine 5xe2x80x2-monophosphate disodium salt, adenosine 5xe2x80x2-monophosphate dipotassium salt and adenosine 5xe2x80x2-monophosphate calcium salt.
In an alternate embodiment of the present invention the catalyst is an alkali metal or an alkaline earth metal salt of 2-deoxy-6-phosphogluconic acid having structure IV 
wherein A and p are defined as in structure I and X+ is a charge balancing species. In one aspect of the present invention X+ is a charge balancing species selected from the group consisting of a proton, an alkali metal cation, an alkaline earth metal cation, an ammonium cation, and a phosphonium cation. Where X+ is a polyvalent cationic species such as an alkaline earth metal ion, it is understood that the positive and negative charges are completely balanced. Thus in instances in which A in structure IV is sodium, p in structure IV is 2 and X in structure IV is magnesium, it is understood that X+ represents 2(Mg+2). Alkali metal and alkaline earth metal salts of 2-deoxy-6-phosphogluconic acid having structure IV are illustrated by 2-deoxy-6-phosphogluconic acid trisodium salt, 2-deoxy-6-phosphogluconic acid disodium salt, 2-deoxy-6-phosphogluconic acid trilithium salt, 2-deoxy-6-phosphogluconic acid tripotassium salt and 2-deoxy-6-phosphogluconic acid disodium magnesium salt.
The present invention relates to salts of organic phosphates useful as catalysts under melt polymerization conditions comprising contacting at least one dihydroxy aromatic compound and at least one diaryl carbonate. Dihydroxy aromatic compounds which are useful in preparing polycarbonates according to the method of the present invention may be represented by the general formula V 
wherein R2 is independently at each occurrence a halogen atom, nitro group, cyano group,C1-C20 alkyl group, C4-C20 cycloalkyl group, or C6-C20 aryl group; n and m are independently integers 0-3; and W is a bond, an oxygen atom, a sulfur atom, a SO2 group, a C1-C20 aliphatic radical, a C6-C20 aromatic radical, a C6-C20 cycloaliphatic radical or the group 
wherein R3 and R4 are independently a hydrogen atom, C1-C20 alkyl group, C4-C20 cycloalkyl group, or C4-C20 aryl group; or R3 and R4 together form a C4-C20 cycloaliphatic ring which is optionally substituted by one or more C1-C20 alkyl, C6-C20 aryl, C5-C2, aralkyl, C5-C20 cycloalkyl groups or a combination thereof.
Suitable bisphenols V according to the method of the present invention include bisphenol A; 2,2-bis(4-hydroxy-3-methylphenyl)propane; 2,2-bis(3-chloro-4-hydroxyphenyl)propane; 2,2-bis(3-bromo-4-hydroxyphenyl)propane; 2,2-bis(4-hydroxy-3-isopropylphenyl)propane; 1,1-bis(4-hydroxyphenyl)cyclohexane; 1,1-bis(4-hydroxy-3-methylphenyl)cyclohexane; and 1,1-bis(4-hydroxyphenyl)-3,3,5-trimethylcyclohexane.
In one aspect of the present invention, the diaryl carbonate used according to the method of the present invention has structure VI 
wherein R5 is independently at each occurrence a halogen atom, nitro group, cyano group, C1-C20 alkyl group, C1-C20 alkoxy carbonyl group, C4-C20 cycloalkyl group, or C6-C20 aryl group; and r and s are independently integers 0-5.
Diaryl carbonates VI suitable for use according to the method of the present invention are illustrated by diphenyl carbonate, bis(4-methylphenyl) carbonate, bis(4-chlorophenyl) carbonate, bis(4-fluorophenyl) carbonate, bis(2-chlorophenyl) carbonate, bis(2,4-difluorophenyl) carbonate, bis(4-nitrophenyl) carbonate, bis(2-nitrophenyl) carbonate, and bis(methyl salicyl) carbonate (CAS No. 82091-12-1).
In one embodiment, structural units of the product polycarbonate derived from the dihydroxy aromatic compound are comprised entirely of structural units derived from bisphenol A, and structural units derived from the diaryl carbonate are derived entirely from diphenyl carbonate.
Optionally, one or more branching agents may be included during the melt polymerization reaction according to the method of the present invention as a means of effecting the controlled branching of the product polycarbonate as is sometimes desirable in applications, such as in blow molding of beverage bottles, requiring a high degree level of melt strength. Suitable branching agents include 1,1,1-tris(4-hydroxyphenyl)ethane (THPE) and 1,3,5-trihydroxybenzene.
In the process of the present invention, an endcapping agent may optionally be used. Suitable endcapping agents include hydroxy aromatic compounds such as phenol, p-tert-butylphenol, p-cumylphenol, cardanol and the like.
When an endcapping agent is employed said endcapping agent is preferably used in an amount corresponding to between about 0.001 and about 0.10 moles, preferably about 0.01 to about 0.08 moles per mole of the dihydroxy aromatic compound employed.
The catalyst having structure I is employed in an amount corresponding to between about 1xc3x9710xe2x88x928 and 2.5xc3x9710xe2x88x924, preferably between about 1xc3x9710xe2x88x927 and 2.5xc3x9710xe2x88x925 moles of catalyst per mole of dihydroxy aromatic compound employed. When the amount of catalyst employed is less than 1xc3x9710xe2x88x928 mole of catalyst per mole of dihydroxy aromatic compound employed, reaction rates may be reduced to such an extent that no appreciable molecular weight gain is observed. Generally, it is preferred that the number average molecular weight (Mn) of the product polycarbonate be at least about 8000 daltons. When the amount of catalyst is in excess of about 2.5xc3x9710xe2x88x924 moles per mole of dihydroxy aromatic compound employed, rates of Fries rearrangement may be excessive and high levels of uncontrolled branching may occur.
The transesterification catalyst may be added in a variety of forms according to the method of the present invention. The catalyst may be added as a solid, for example a powder, or it may be dissolved in a solvent, for example water or alcohol. In one embodiment, the catalyst is added to the reaction system in the form of an aqueous solution. The pH of the aqueous solution is preferably at or near the pH of a freshly prepared solution, which varies depending on the identity of the catalyst used.
If the catalyst is not soluble in water or only sparingly soluble in water, the catalyst may be dissolved in a solution comprising a quaternary ammonium compound, a quaternary phosphonium compound, or a mixture thereof, and the solution comprising the catalyst and said onium compound may be used.
The use of a quaternary ammonium compound, a quaternary phosphonium compound, or a mixture thereof, as a co-catalyst is not limited to instances in which the catalyst has low solubility and is introduced into the reaction mixture as an aqueous solution. Quaternary ammonium compounds, quaternary phosphonium compounds, and mixtures thereof, may be employed as co-catalysts and may be introduced into the reaction mixture in a variety of forms; as solids, in solution, or as a melt. Examples of suitable quaternary ammonium compounds include, but are not limited to ammonium hydroxides having alkyl groups, aryl groups and alkaryl groups, such as tetraamethylammonium hydroxide (TMAH) and tetrabutylammonium hydroxide (TBAH). Suitable phosphonium compounds include, but are not limited to tetraethylphosphonium hydroxide, tetrabutylphosphonium hydroxide, and tetrabutylphosphonium acetate.
If present, the quaternary ammonium compound, the quaternary phosphonium compound, or a mixture thereof, is preferably used in amounts of from about 1xc3x9710xe2x88x922 to about 1xc3x9710xe2x88x926, preferably about 1xc3x9710xe2x88x922 to about 1xc3x9710xe2x88x925 moles per mole of dihydroxy aromatic compound. The quaternary onium compounds may be used to enhance or modify the activity of the organic phosphate salt derivative employed as the primary catalyst.
In some instances the reaction mixture may further comprise a metal hydroxide, for example, an alkali metal hydroxide such as sodium hydroxide. The free metal hydroxide may be added to enhance the activity of the alkali metal or alkaline earth metal salt of the organic phosphate employed as the primary catalyst, or may be present as a contaminant in the primary catalyst itself. If present, the free metal hydroxide is preferably present in an amount in an amount corresponding to between about 1xc3x9710xe2x88x928 and 2.5xc3x9710xe2x88x924 moles of catalyst per mole of dihydroxy aromatic compound employed.
The reaction conditions of the melt polymerization are not particularly limited and may be conducted in a wide range of operating conditions. Hence, the term xe2x80x9cmelt polymerization conditionsxe2x80x9d will be understood to mean those conditions necessary to effect reaction between the diaryl carbonate and the dihydroxy aromatic compound of the present invention. The reaction temperature is typically in the range of about 100 to about 350xc2x0 C., more preferably about 180 to about 310xc2x0 C. The pressure may be at atmospheric pressure, supraatmospheric pressure, or a range of pressures from atmospheric pressure to about 15 torr in the initial stages of the reaction, and at a reduced pressure at later stages, for example in the range of about 0.2 to about 15 torr. The reaction time is generally about 0.1 hours to about 10 hours.
The melt polymerization may be accomplished in one or more stages, as is known in the art with other catalysts. The catalyst and co-catalysts of the present invention may be added in the same stage or different stages, if the melt polymerization is conducted in more than one stage. The optional co-catalyst may be added at any stage, although it is preferred that it be added early in the process. When utilized, the co-catalyst is preferably utilized in an amount corresponding to between about 1 and about 500 molar equivalents, based on the moles of catalyst I utilized.
In a further preferred embodiment, the process is conducted as a two stage process. In the first stage of this embodiment, the optional co-catalyst of the present invention is introduced into the reaction system comprising the dihydroxy aromatic compound and the diaryl carbonate. The first stage is conducted at a temperature of 270xc2x0 C. or lower, preferably 80 to 250xc2x0 C., more preferably 100 to 230xc2x0 C. The duration of the first stage is preferably 0 to 5 hours, even more preferably 0 to 3 hours at a pressure from about atmospheric pressure to about 100 torr, with a nitrogen atmosphere preferred.
In a second stage, the catalyst having structure I is introduced into the product from the first stage and further polycondensation is conducted. The catalyst may be added in its entire amount in the second stage, or it may be added in batches in the second and subsequent stages so that the total amount is within the aforementioned ranges.
It is preferable in the second and subsequent stages of the polycondensation step for the reaction temperature to be raised while the reaction system is reduced in pressure compared to the first stage, thus bringing about a reaction between the dihydroxy aromatic compound and the diaryl carbonate. Thus there is formed initially a polycarbonate oligomer which upon further polycondensation reaction at 240 to 320xc2x0 C. under reduced pressure of 5 mm Hg or less, and preferably 1 mm Hg or less, affords polycarbonate having a number average molecular weight of about 8000 daltons or greater.
If the melt polymerization is conducted in more than one stage, as noted above, it is preferable to add a co-catalyst, such as tetramethylammonium hydroxide, tetrabutylammonium hydroxide, tetrabutylphosphonium hydroxide, or tetrabutylphosphonium acetate, in an earlier stage than the catalyst of the present invention. In particular, it is preferable to add the co-catalyst to the reactor before the temperature reaches 220xc2x0 C., preferably before it reaches 200xc2x0 C.
The reaction can be conducted as a batch or a continuous process. Any desired apparatus can be used for the reaction. The material and the structure of the reactor used in the present invention is not particularly limited as long as the reactor has an ordinary capability of stirring and is equipped for the removal of by-product hydroxy aromatic compound formed during the course of the polymerization. It is preferable that the reactor is capable of stirring in high viscosity conditions as the viscosity of the reaction system is increased in later stages of the reaction.
Thus, in a further embodiment, the present invention provides a method for preparing polycarbonates, which comprises the steps of
(a) heating a dihydroxy aromatic compound and a diaryl carbonate for a time period sufficient to form a melt, and thereafter introducing a catalyst composition comprising a catalytically effective amount of a compound having structure I and optionally a co-catalyst selected from tetraalkylammonium and tetraalkylphosphonium compounds;
(b) oligomerizing the melt mixture formed in step (a) in a reaction system comprising at least one continuous reactor in series, wherein said reactor is operated at a temperature of about 210xc2x0 C. to about 290xc2x0 C., and wherein the product from the reactor has a number average molecular weight of about 1000 to about 5500 daltons; and
(c) polymerizing the product from step (b) in a reaction system comprising at least one continuous polymerization reactor in series, wherein said reactor is operated at a temperature of about 280xc2x0 C. to 315xc2x0 C., wherein the product from step (c) has a number average molecular weight of at least about 8000 daltons.
Additives may also be added to the polycarbonate product as long as they do not adversely affect the properties of the product. These additives include a wide range of substances that are conventionally added to the polycarbonates for a variety of purposes. Specific examples include heat stabilizers, epoxy compounds, ultraviolet absorbers, mold release agents, colorants, antistatic agents, slipping agents, anti-blocking agents, lubricants, antifogging agents, natural oils, synthetic oils, waxes, organic fillers, flame retardants, inorganic fillers and any other commonly known class of additives.
The polycarbonate obtained in accordance with the present invention may be used after being mixed with conventional additives, such as plasticizers, pigments, lubricants, mold release agents, stabilizers and organic fillers. It is also possible to blend the polycarbonate with other polymers, including but not limited to, olefins polymers such as ABS and polystyrenes, polyesters, polyacrylates, polyethersulfones, polyamides, and polyphenylene ethers.